Millets: genetic and genomic resources

Abstract

all-grained millets, comprising ten annual grasses from the family Poaceae and grown for grain, contribute $13% of annual global cereal production. Some are widely grown, while cultivation of others is restricted. They differ in ploidy, genome size, and breeding system, but their grains are all highly nutritious. Their most common nonfood uses are in brewing and as livestock feeds. Millets are C4 plants adapted to marginal lands in hot, drought-prone arid and semiarid regions. Selection for plant phenology and architecture, panicle shape, spikelet structure and reduced shattering, seed dormancy, and seed coat hardness contributed to their domestication. Approximately 161,708 millet accessions are preserved in gene banks globally. These show exceptional diver- sity associated for phenology, photoperiod sensitivity, tolerance to abiotic stresses, resistance to biotic stresses, seed storability and shelf life, and specific grain characteristics associated with end user preferences. Contributions from wild relatives’ toward enhancing cultivated gene pools have been limited to pearl millet and foxtail millet. Core or minicore/reference collections have been used to identify new sources of biotic stress resistances and abiotic stress tolerances. Waxy mutants have been selected in barnyard millet, foxtail millet, and proso millet for specific food uses. Pearl millet hybrids and open pollinated varieties (OPVs) with high iron and zinc grain densities will soon be available in India. While no transgenic work has reached field level, DNA markers are routinely used to assess millets’ population structure and genetic diversity. Genetic maps of varying density are reported in finger millet, foxtail millet, pearl millet, proso millet, and tef. Major quantitative trait loci associated with resis- tance to downy mildew, rust, and blast and tolerance to terminal drought stress have been backcrossed into elite inbred pearl millet hybrid parents. Marker- assisted backcrossing has been used to improve downy mildew resistance in pearl millet. Cytoplasmic-genetic male sterility (CMS)–based hybrids of pearl millet are extensively cultivated, and CMS systems for foxtail millet are under development. An aligned genome sequence of foxtail millet will be released in the near future as this millet is closely related to several polyploid bioenergy grasses. This foxtail millet genome sequence is highly syntenic with those of rice, sorghum, and maize, which should allow comprehensive surveys of genetic diversity for identifying and conserving diversity in grass germplasm with bioenergy crop potential.